Some transmission type image display apparatuses such as liquid crystal display apparatuses use a direct surface light source device as a backlight. In recent years, direct surface light source devices having a plurality of light emitting elements as the light source have been used (see, for example, PTL 1).
FIG. 1 is a plan view of surface light source device 10 disclosed in PTL 1. As illustrated in FIG. 1, surface light source device 10 disclosed in PTL 1 includes a plurality of LEDs (light emitting elements) 12, a plurality of LED substrates 14 on which the plurality of LEDs 12 are disposed, reflection sheet 16 disposed on LED substrates 14 and including a plurality of openings disposed at positions corresponding to positions of the plurality of LEDs, and backlight chassis 18 configured to house the above-mentioned parts. The edge of reflection sheet 16 is obliquely raised with respect to the bottom plate. In surface light source device 10 disclosed in PTL 1, light emitted from LED 12 and reached reflection sheet 16 is reflected, and it is thus possible to illuminate a region around an illumination region without providing LED 12 at a peripheral portion of reflection sheet 16. However, in surface light source device 10 disclosed in PTL 1, disadvantageously, a part immediately above LED 12 (a center portion of the illumination region) is bright, but the brightness at four corners of the illumination region is insufficient.
To solve this problem, techniques of adjusting the orientations of LEDs, diffusion lenses and the like have been proposed (see, for example, PTL 2). PTL 2 discloses a surface light source device including: a plurality of LEDs (light emitting elements), a plurality of diffusion lenses configured to control a distribution of light emitted from the LEDs, a plurality of LED substrates on which the plurality of LEDs and the plurality of diffusion lenses are disposed, a reflection sheet disposed on the LED substrates and including a plurality of openings disposed at positions corresponding to the positions of the plurality of LEDs and the plurality of diffusion lenses, a diffusion plate and an optical sheet disposed with an air layer interposed with the diffusion lenses, and a backlight chassis configured to house the above-mentioned parts. The plurality of diffusion lenses and the plurality of LEDs disposed on the outside are formed such that the optical axis of the LED and the central axis of the emission surface of the diffusion lens are oriented to the external edge of the diffusion plate and the optical sheet. In the surface light source device disclosed in PTL 2, the optical axis of the LED disposed at the center and the central axis of the emission surface of the diffusion lens are oriented to the center portion of the illumination region (immediate upper part), and the central axis of the emission surface of the diffusion lens and the optical axis of the LED disposed on the outside are oriented toward the external edge of the illumination region, and, it is thus possible to uniformly illuminate the illumination region.